If you use Blend or any of the interactivity extensions (for triggers and behaviors) be sure to get the latest version of Blend

If you use the Toolkit, download and install the latest version of that as well

Open your project in Visual Studio 2010, then

Right-click the Silverlight projects, go into properties and change to target Silverlight 4

Delete any bad Toolkit or Blend references and re-add them to point to the current version

If you used the MEF preview, the references should update automatically — delete any local copies of the DLLs you may have included in the project

Compile and run: in most cases, you're done!

In this simple example, I not only converted the application, but took advantage of the fact that buttons support commanding and removed a trigger I created in order to bind the button directly. Enjoy!